Conventionally, shot peening has been known as a useful method to enhance the fatigue strength of a high-strength steel such as a carburized steel, which is used for gears for automobiles, etc. A compressive residual stress in the surface layer that is generated by shot peening is known to significantly affect the bending fatigue strength at the root of a tooth.
It is also well known that the compressive residual stress is affected by the sizes, hardnesses, shooting speeds, shooting times, etc. of the shot materials. Many studies have been made about the effects of the shot-peening conditions on the compressive residual stress.
Recently, needs for higher-strength steels have increased as components are made smaller. Accordingly generating a higher compressive residual stress in a processed material by shot peening is required to achieve a higher fatigue strength.
For example, to achieve a higher fatigue strength by 20%, a compressive residual stress at 1800 MPa in a processed material is required when the peak compressive residual stress that is generated by current heavy shot peening is 1500 MPa.
Previously, developing harder shot materials has been the main way to achieve the higher compressive residual stress in the processed material. However, shot peening harder shot materials does not always cause the processed material to generate a higher compressive residual stress. In fact, it may adversely decrease the compressive residual stress. The hardness of the shot materials must be appropriate for the processed material.
For example, in some combinations of shot materials having a certain hardness and a processed material having a certain hardness, the processed material may be significantly scraped by the shot materials. In this case, the energy for shooting is wasted in scraping. Thus no compressive residual stress is effectively generated in the processed material.
If the shot materials have a much higher hardness than the processed material, a high compressive residual stress is generated, but much of the processed material is scraped. Thus the roughness of the surface of the processed material becomes coarse. That may create a point for initiating a fatigue fracture. Further, a large amount to be scraped may result in decreasing the size of a component.
Shot materials that have a significantly higher hardness are expensive. Even if shot materials that are expensive are used, the compressive residual stress that is generated in the processed material would not increase over a certain value. Thus, only the cost would increase.
Therefore it is important to balance the hardness of the shot materials with that of the processed material to properly generate a higher compressive residual stress in the surface layer of the processed material.
Until now no finding has been disclosed for such ways of thinking. For example, techniques to generate a compressive residual stress in a processed material by shooting the shot materials against the processed material were disclosed in Japanese Patent Laid-open Publication No. 2002-36115, Japanese Patent Laid-open Publication No. 2001-79766, and Japanese Patent Laid-open Publication No. H9-57629.
However, Japanese Patent Laid-open Publication No. 2002-36115 does not discuss scraping. Japanese Patent Laid-open Publication No. 2001-79766 does not discuss any relationship between a processed material and shot materials, nor does Japanese Patent Laid-open Publication No. H9-57629.